Pressure-temperature Plane Research Articles

Assembly of clathrates from tetrahedral patchy colloids with narrow patches.

Here, we revisit the assembly of colloidal tetrahedral patchy particles. Previous studies have shown that the crystallization of diamond from the fluid phase depends more critically on patch width than on the interaction range: particles with patches narrower than 40° crystallize readily and those with wide patches form disordered glass states. We find that the crystalline structure formed from the fluid also depends on the patch width. Whereas particles with intermediate patches assemble into diamond (random stacking of cubic and hexagonal diamond layers), particles with narrow patches (with width ≈20° or less) crystallize frequently into clathrates. Free energy calculations show that clathrates are never (in the pressure-temperature plane) thermodynamically more stable than diamond. The assembly of clathrate structures is thus attributed to kinetic factors that originate from the thermodynamic stabilization of pentagonal rings with respect to hexagonal ones as patches become more directional. These pentagonal rings present in the fluid phase assemble into sII clathrate or into large clusters containing 100 particles and exhibiting icosahedral symmetry. These clusters then grow by interpenetration. Still, the organization of these clusters into extended ordered structures was never observed in the simulations.

Discrete potential fluids in the supercritical region

Supercritical fluids are an interesting area of research because their use in industrial processes has environmental benefits. Since their processes are low energy consumption they are considered as green solvents of the future. However, the study of the supercritical region remains a controversial issue, particularly, the possibility to find three different regime: gas-like, liquid-like and a mixture of them. The existence of a line (Widom line) or a region (delta Widom) that delimit this behavior is still under discussion. In this work we assume that this border is a line, like a continuation of a vapor-liquid saturation curve in the pressure-temperature plane. The Widom line is defined by the locus of points of maximum correlation length. Taking advantage of the relation that exists between the correlation length with the response functions and scalar curvature in the vicinity of the critical point, we build the Widom line with the locus of extrema of response functions and scalar curvature, and use the coincidence of these curves to determine the end of the Widom line. The Widom lines for Square-Well of variable range and Hard-Core Lennard-Jones systems are obtained using analytical equations of state. We propose a quantitative criterion to determine the end of this line. Besides, we give a new and alternative way of determining this line using the locus of the pressure inflection points (obtained in the density-pressure plane) and look for the coincidence of these points with the locus of the maxima of the isothermal compressibility in the temperature-pressure plane.

Experimental Evidence for a State-Point-Dependent Density-Scaling Exponent of Liquid Dynamics.

A large class of liquids obey density scaling characterized by an exponent, which quantifies the relative roles of temperature and density for the dynamics. We present experimental evidence that the density-scaling exponent γ is state-point dependent for the glass formers tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl ether (5PPE). A method is proposed that from dynamic and thermodynamic properties at equilibrium estimates the value of γ. The method applies at any state point of the pressure-temperature plane, both in the supercooled and the normal liquid regimes. We find that γ is generally state-point dependent, which is confirmed by reanalyzing data for 20 metallic liquids and two model liquids.

Pressure-Temperature Analysis of the Stability of the CTL9 Domain Reveals Hidden Intermediates

The observation of two-state unfolding for many small single-domain proteins by denaturants has led to speculation that protein sequences may have evolved to limit the population of partially folded states that could be detrimental to fitness. How such strong cooperativity arises from a multitude of individual interactions is not well understood. Here, we investigate the stability and folding cooperativity of the C-terminal domain of the ribosomal protein L9 in the pressure-temperature plane using site-specific NMR. In contrast to apparent cooperative unfolding detected with denaturant-induced and thermal-induced unfolding experiments and stopped-flow refolding studies at ambient pressure, NMR-detected pressure unfolding revealed significant deviation from two-state behavior, with a core region that was selectively destabilized by increasing temperature. Comparison of pressure-dependent NMR signals from both the folded and unfolded states revealed the population of at least one invisible excited state at atmospheric pressure. The core destabilizing cavity-creating I98A mutation apparently increased the cooperativity of the loss of folded-state peak intensity while also increasing the population of this invisible excited state present at atmospheric pressure. These observations highlight how local stability is subtly modulated by sequence to tune protein conformational landscapes and illustrate the ability of pressure- and temperature-dependent studies to reveal otherwise hidden states.

Application of near critical behavior of equilibrium ratios to phase equilibrium calculations

We examine the asymptotic behavior of the equilibrium ratios (Ki) near the convergence locus in the pressure-temperature plane. When the Equation of State (EoS) is analytical, which is the case of most EoS of engineering purpose, Ki tends towards unity or, equivalently, its logarithm lnKi tends to zero, according to a power ½ of the distance to this locus. As a consequence, if lnKi is expressed as a linear combination of pure component parameters with coefficients only depending on mixture phase properties (i.e., reduction parameters), these coefficients obey a similar power law. Deviations from the ½ power law are thus fairly limited for lnKi and for the reduction parameters (at least in the negative flash window between the convergence locus and the phase boundaries), which can be exploited to speed up flash calculations and for quickly determining approximate saturation points and convergence pressures and temperatures. The chosen examples are representative synthetic and natural hydrocarbon mixtures, as well as various injection gas-hydrocarbon systems.

Density-based phase envelope construction

A new density-based phase envelope construction procedure is proposed. The equations for two-phase saturation molar density and temperature computations are established by equating to zero the tangent plane distance (TPD) function in terms of component molar densities and temperature to find a non-trivial set of component molar densities at various specifications: mixture molar density, temperature and the modified equilibrium constants. The latter are defined as the ratios of feed to incipient phase component molar density. The nonlinear system of equations is solved by the full Newton method. The sequence of calculations is started at some conditions where convergence is easily obtained and an extrapolation procedure provides high quality initial guesses for subsequent calculations. The phase envelope is traced in the molar density-temperature plane, where a unique saturation temperature exists at specified mixture molar density; the representation in the pressure-temperature plane is obtained by calculating explicitly the pressure from the equation of state at given temperature and component molar densities on the saturation curve. The equation of state (EoS) must not be solved for volume and the elements of the Jacobian matrix have simpler expressions than their conventional (pressure-based) counterparts. The proposed method is successfully tested for a variety of mixtures, ranging from binary and ternary mixtures to reservoir fluids, exhibiting usual (closed) and unusual phase envelopes, such as open-shaped envelopes characterized by a bubble point branch extending to infinity, branches with negative pressures constructed in a single run (unlike in the conventional case), several branches, double retrograde behavior and swallowtail pattern corresponding to liquid demixing at low temperatures. The proposed method is not dependent of the thermodynamic model and any pressure-explicit equation of state can be used, provided the required partial derivatives of fugacity and pressure with respect to mole numbers, temperature and volume are available.

Mathematical model of formation of carbon dioxide hydrate upon injection of carbon dioxide into a methane hydrate stratum

A mathematical model of formation of carbon dioxide gas hydrate upon injection of warm carbon dioxide into a natural stratum saturated with methane and methane hydrate has been presented. The case when methane hydrate decomposes into gas and water on two frontal boundaries and the subsequent formation of carbon dioxide hydrate from carbon dioxide and water has been discussed. The regions where this mode is implemented depending on stratum permeability have been studied based on the pressure–temperature plane of the gas being injected into the stratum.

Onset of quantum criticality in the topological-to-nematic transition in a two-dimensional electron gas at filling factor ν=5/2

Under hydrostatic pressure, the ground state of a two-dimensional electron gas at $\nu=5/2$ changes from a fractional quantum Hall state to the stripe phase. By measuring the energy gap of the fractional quantum Hall state and of the onset temperature of the stripe phase we mapped out a phase diagram of these competing phases in the pressure-temperature plane. Our data highlight the dichotomy of two descriptions of the half-filled Landau level near the quantum critical point: one based on electrons and another on composite fermions.

Communication: Re-entrant limits of stability of the liquid phase and the Speedy scenario in colloidal model systems.

A re-entrant gas-liquid spinodal was proposed as a possible explanation of the apparent divergence of the compressibility and specific heat off supercooling water. Such a counter-intuitive possibility, e.g., a liquid that becomes unstable to gas-like fluctuations on cooling at positive pressure, has never been observed, neither in real substances nor in off-lattice simulations. More recently, such a re-entrant scenario has been dismissed on the premise that the re-entrant spinodal would collide with the gas-liquid coexisting curve (binodal) in the pressure-temperature plane. Here we study, numerically and analytically, two previously introduced one-component patchy particle models that both show (i) a re-entrant limit of stability of the liquid phase and (ii) a re-entrant binodal, providing a neat in silico (and in charta) realization of such unconventional thermodynamic scenario.

Gas-containing semiclathrate hydrate formation by tetra-n-butylammonium carboxylates: Acrylate and butyrate

Semiclathrate hydrates are water-based host-guest materials that consist of water and ionic guest substances such as quaternary ammonium salts. Widely-used tetra-n-butylammonium (TBA) salts are designed for gas selectivity and capacity with respect to the industrial application of these materials. This study reported on the formation of semiclathrate hydrates of TBA carboxylates with guest gas characterized by phase equilibrium measurements and single crystal X-ray diffraction. Specifically, TBA acrylate and TBA butyrate are used for ionic guest substances, and this may induce a tetragonal P42/mmm semiclathrate hydrate structure. In the four systems of (TBA acrylate or TBA butyrate) + (CH4 or CO2) + H2O, the measured three (gas–hydrate–aqueous) phase equilibrium data exhibited better promotion effects due to the TBA acrylate and TBA butyrate when compared with that of the widely-used TBA bromide. The slopes on pressure–temperature plane suggest differences in the incorporation of CH4 and CO2 in these hydrates. Characterization of the hydrate phases by X-ray diffraction analyses supports this mechanism. However, it is necessary to further understand the complicated gas incorporation in semiclathrate hydrate structures that are handled by ionic guest substances and the guest gas itself.

Investigation of critical lines and global phase behavior of unequal size of molecules in binary gas–liquid mixtures in the combined pressure–temperature–concentration planes around the van Laar point

Abstract We investigate critical curves and global phase behavior of unequal size of molecules in binary gas–liquid mixtures at the van Laar point and its vicinity. The van Laar point is only point at which the mathematical double point curve is stable, and also the intersection of the tricritical point and the double critical end point. The critical line structure is displayed for various combinations of the chain length and system parameters in the reduced pressure ( P ∗ ) temperature ( T ∗ ) plane, as is usually done with experimental results and temperature–concentration ( T , x ) plane. The P ∗ , T ∗ diagrams are discussed in accordance with the Scott and van Konynenburg binary phase diagram classification. We found that our P ∗ , T ∗ plots correspond to the type II, type III, type IV phase diagram behaviors and they are in good agreement with the theoretical and experimental studies. It is also found that the critical lines and phase behavior are extremely sensitive to small modifications in the system parameters.

The Effect of Temperature Changes in Vitreoretinal Surgery.

PurposeRecent studies on temperature control in biology and medicine have found the temperature as a new instrument in healthcare. In this manuscript, we reviewed the effects of temperature and its potential role in pars plana vitrectomy. We also examined the relationship between intraocular pressure, viscosity, and temperature in order to determine the best balance to manipulate the tamponades during the surgery.MethodsA literature review was performed to identify potentially relevant studies on intraocular temperature. Physics equations were applied to explain the described effects of temperature changes on the behavior of the endotamponades commonly used during vitreoretinal surgery. We also generated an operating diagram on the pressure–temperature plane for the values of both vapor–liquid equilibrium and intraocular pressure.ResultsThe rapid circulation of fluid in the vitreous cavity reduces the heat produced by the retinal and choroidal surface, bringing the temperature toward room temperature (22°C, deep hypothermia). Temperature increases with endolaser treatment, air infusion, and the presence of silicone oil. The variations in temperature during vitreoretinal surgery are clinically significant, as the rheology of tamponades can be better manipulated by modulating intraocular pressure and temperature.ConclusionsDuring vitreoretinal surgery, the intraocular temperature showed rapid and significant fluctuations at different steps of the surgical procedure inside the vitreous cavity. Temperature control can modulate the rheology of tamponades.Translational RelevanceIntraoperative temperature control can improve neuroprotection during vitreoretinal surgery, induce the vaporization of perfluorcarbon liquid, and change the shear viscosity of silicone oil.

Challenge for methane hydrate production by geotechnical engineering

Methane hydrate (referred to as MH hereafter) has come to be seen as a future energy source and as such research and development is being conducted in order to prepare for production of it. The MH reservoirs are located in the deep sea bed at over 1000m in sea depth. In Japan, the reservoirs are found in the Nankai Trough on the Pacific Ocean side of main island. MH mostly exists in the void space of the sand layers in the turbidite. In order to produce MH from the MH bearing layers, it is important to first make clear and understand the mechanical properties of the host sands of MH bearing sands in order to understand the MH bearing layer. A series of mechanical tests were performed on MH bearing sediments with various host sands by using temperature controlled high stress triaxial shear testing apparatus. A high pressure and low temperature plane strain testing apparatus was also developed for visualizing the deformation of methane hydrate bearing sand due to methane hydrate production. Using this testing apparatus, plane strain compression and methane hydrate dissociation by depressurization tests were performed with the measurement of localized deformation. On the basis of the experimental results, an elastoplastic constitutive model for methane hydrate-bearing sand was developed. The bonding effect due to the cementation by methane hydrate was introduced into the model. The simulation was performed on triaxial behaviour of methane-hydrate bearing sand under various pressure and temperature condition.

Contribution of Water to Pressure and Cold Denaturation of Proteins.

The mechanisms of cold and pressure denaturation of proteins are matter of debate and are commonly understood as due to water-mediated interactions. Here, we study several cases of proteins, with or without a unique native state, with or without hydrophilic residues, by means of a coarse-grain protein model in explicit solvent. We show, using MonteCarlo simulations, that taking into account how water at the protein interface changes its hydrogen bond properties and its density fluctuations is enough to predict protein stability regions with elliptic shapes in the temperature-pressure plane, consistent with previous theories. Our results clearly identify the different mechanisms with which water participates to denaturation and open the perspective to develop advanced computational design tools for protein engineering.

Water Mediated Interactions and the Protein Folding Phase Diagram in the Temperature-Pressure Plane.

The temperature-pressure behavior of two proteins, ubiquitin and λ-repressor, is explored using a realistically coarse-grained physicochemical model, the associative memory, water mediated, structure and energy model (AWSEM). The phase diagram across the temperature-pressure plane is obtained by perturbing the water mediated interactions in the Hamiltonian systematically. The phase diagrams calculated with direct simulations along with an extended bridge sampling estimator show the main features found experimentally, including both cold- and pressure-denaturation. The denatured ensembles in different parts of the phase diagram are characterized and found to be structurally distinct. The protein energy landscape is found to be funneled throughout the phase diagram, but modest changes in the entropy and free energy of the water are found to drive both cold and pressure induced denaturation.

Phase diagram of the ST2 model of water

We evaluate the free energy of the fluid and crystal phases for the ST2 potential with reaction field corrections for the long-range interactions. We estimate the phase coexistence boundaries in the temperature–pressure plane, as well as the gas–liquid critical point and gas–liquid coexistence conditions. Our study frames the location of the previously identified liquid–liquid critical point relative to the crystalline phase boundaries, and opens the way for exploring crystal nucleation in a model where the metastable liquid–liquid critical point is computationally accessible.

Liquid-Liquid Phase Transition and Its Phase Diagram in Deeply-Cooled Heavy Water Confined in a Nanoporous Silica Matrix.

Using neutron diffraction technique, we measure the average density of the heavy water confined in a nanoporous silica matrix, MCM-41, over the pressure-temperature plane. The result suggests the existence of a line of liquid-liquid phase transition with its end point at 1.29 ± 0.34 kbar and 213 ± 3 K in a fully hydrated sample. This point would be the liquid-liquid critical point (LLCP) according to the "liquid-liquid critical point" scenario. The phase diagram of the deeply cooled confined heavy water is then discussed. Moreover, in a partially hydrated sample, the phase transition completely disappears. This result shows that it is the free water part, rather than the bound water part, of the confined water that undergoes a liquid-liquid transition.

Temperature and pressure dependence of methane correlations and osmotic second virial coefficients in water.

We report methane's osmotic virial coefficient over the temperatures 275 to 370 K and pressures from 1 bar up to 5000 bar evaluated using molecular simulations of a united-atom description of methane in TIP4P/2005 water. In the first half of this work, we describe an approach for calculating the water-mediated contribution to the methane-methane potential-of-mean force over all separations down to complete overlap. The enthalpic, entropic, heat capacity, volumetric, compressibility, and thermal expansivity contributions to the water-mediated interaction free energy are subsequently extracted from these simulations by fitting to a thermodynamic expansion over all the simulated state points. In the second half of this work, methane's correlation functions are used to evaluate its osmotic second virial coefficient in the temperature-pressure plane. The virial coefficients evaluated from the McMillan-Mayer correlation function integral are shown to be in excellent agreement with those determined from the concentration dependence of methane's excess chemical potential, providing an independent thermodynamic consistency check on the accuracy of the procedures used here. At atmospheric pressure the osmotic virial coefficient decreases with increasing temperature, indicative of increasing hydrophobic interactions. At low temperature, the virial coefficient decreases with increasing pressure while at high temperature the virial coefficient increases with increasing pressure, reflecting the underlying hyperbolic dependence of the virial coefficient on temperature and pressure. The transition between a decreasing to increasing pressure response of the osmotic virial coefficient is shown to follow the response of the methane-methane contact peak to changes in pressure as a function of temperature, though a universal correlation is not observed.

Thermodynamics, compressibility, and phase diagram: shock compression of supercritical fluid xenon.

Supercritical fluids have intriguing behaviors at extreme pressure and temperature conditions, prompting the need for thermodynamic properties of supercritical fluid xenon (SCF) under shock compression. Double-shock experimental data on SCF xenon in the 140 GPa pressure range were directly measured by means of a multi-channel pyrometer and a Doppler-pins-system. It entered the so-called warm dense region. We found that the shock compressed SCF Xe had higher dynamic compression and higher number density than that of liquid Xe at same shock pressure. The larger compressibility of SCF Xe in our experiments could be explained that the increase of electronic excitations and ionizations leaded to a large drop of thermal pressure and a softening of Hugoniot. The high pressure phase diagram of xenon was depicted with the aid of the degeneracy, coupling parameter, and current available experiments on the pressure-temperature plane.

Evidence of the existence of the high-density and low-density phases in deeply-cooled confined heavy water under high pressures.

The average density of D2O confined in a nanoporous silica matrix (MCM-41-S) is studied with neutron scattering. We find that below ~210 K, the pressure-temperature plane of the system can be divided into two regions. The average density of the confined D2O in the higher-pressure region is about 16% larger than that in the lower-pressure region. These two regions could represent the so-called "low-density liquid" and "high-density liquid" phases. The dividing line of these two regions, which could represent the associated 1st order liquid-liquid transition line, is also determined.